October 8, 2012

Europe’s Wet Summers Blamed On Atlantic Ocean Warming

It has been long known that ocean temperatures play a significant role in influencing weather patterns around the world. Therefore, it is no surprise that a recent barrage of wet summers that have plagued northern Europe is being blamed on substantial warming of the North Atlantic Ocean through the late 1990s.

A new study carried out by researchers at University of Reading, Berkshire, and published in the journal Nature Geoscience, has found that North Atlantic warming through the 1990s has led to rainier weather systems across much of northern Europe, with rainfall increasing by about a third this past summer.

The 2012 summer in northern Europe was the wettest in a century and follows a string of above average rainy summers. Experts said this trend will continue as long as the Atlantic warming persists.

Lead author Professor Rowan Sutton and colleagues examined more than a century worth of data and found that North Atlantic temperatures remain above or below long-term averages for decades at a time. The periods of warmer temperature, which is the current setting for the North Atlantic, correlate to wetter summers in northern Europe and hotter, drier summers in the Mediterranean.

The phenomenon is known as the Atlantic Multidecadal Oscillation (AMO). The researchers compared three periods of time due to this cycle: a warm period from 1931 to 1960, a cooler period from 1961 to 1990, and the recent cycle which began in the 90s. The researchers noted that the current warming trend is similar to the last warm period that began in the 30s.

The team surmised that the persistent warmth of the North Atlantic tends to make northern and central Europe cooler and wetter than usual. Yet, areas like Portugal, Turkey and other southern European countries receive far less rain than normal, and are noticeably hotter.

The precise mechanisms that drive these weather patterns is not exactly understood. But changes in pressure that influence the jet stream, and how much it “meanders,” is the main theory that researchers have gone with.

Regions to the north of the jet stream, which is where northern Europe was for most of the summer, tend to be on the receiving end of seemingly endless strings of rainy weather systems

“We know that a higher sea surface temperature in the ocean warms the air above which affects the weather systems and their path, shifting the Jetstream, but we don't yet know the full details of how this works,” said Sutton. “We also don't know the length of these periods of warmer or cooler conditions in the Atlantic Ocean - in the past they have varied a lot, from 20-50 years.”

The current pattern is likely to revert again, bringing drier summers to the north, noted Sutton.

“I can't guarantee it but it is likely,” he said. “However we are not sure of the timing, which is what every one wants to know — but we are working on this now.” Sutton added that when the switch occurs, it could happen as rapidly as over two to three years.

However, Sutton´s research did not explore the causes of the warm and cool phases of the Atlantic, they only focused on what it means for weather.

The shifts Sutton and his team discovered have been occurring for many hundreds of years, but global warming could have a lasting impact on the current cycle. “There is lot of evidence to show that climate change is changing the timing and amplitude of the temperature changes.”

For example, he said, the cooler period from the 1960s to the 1980s occurred when soot and other pollution from dirty power stations cooled the planet.

The previous warm phase, which ran from 1931 to 1960, saw an influx of wet summers in the UK, including a severe deluge in August 1948, which closed the east coast mainline railway for three months. Yet, it is currently unknown if manmade climate could have played a role in the warming seen during that cycle.

Experts are also blaming the record drop in Arctic sea ice seen this summer on the warming North Atlantic. It is also possible that the shrinking sea ice could be contributing to the unpleasant summers in the UK, as the exposed ocean waters warm in the sun. However, this has yet to be proven, Sutton remarked, noting that research is currently underway to find a cause and effect.

“These variations are on the decadal time-scale and remind us that the atmosphere and oceans can drive long-term fluctuations in our seasonal weather,” Thorpe told BBC´s David Shukman. “Earlier research at the European Weather Centre had indicated that both the sea surface temperatures and the decreasing summer Arctic sea-ice may in combination have played a role in poor UK summer weather in 2007 and 2008, and one can speculate also in 2012.”

“Overall we can see that there are a number of potential factors that can affect how good or poor the summers are in our part of the world. These factors typically involve far remote influences,” he added.

For the coming summers, Sutton said it is likely the current patterns will continue, as there are no signs of the Atlantic moving toward a cooling trend. Yet, these cycles can change fairly quickly and a transition back to cooler Atlantic waters could happen over a period of just a few years. He noted that cold winters could be a precursor to such a change.

So, noted Sutton, if the coming months bring freezing conditions, that could be our sign that drier summers are on the way.